1. Field of the Invention
The present invention concerns combustion devices and more particularly a method and apparatus for pretreating hydrocarbon fuel prior to its injection into a combustion chamber to a condition which will subsequently cause hypergolic combustion with only negligible delays in ignition and in completing the combustion process.
2. Description of the Prior Art
It has heretofore been proposed to achieve so called "hypergolic" combustion, particularly of hydrocarbon fuels in an internal combustion engine, such that ignition delay and the time required to complete combustion are both negligible after the fuel is introduced into an oxidizing atmosphere.
For a detailed discussion, see U.S. Pat. No. 4,448,176; SAE paper no. 850089 "Hypergolic Combustion in an Internal Combustion Engine"; and, SAE paper 820356, "The Influence of Initial Fuel Temperature on Ignition Delay".
Hypergolic combustion has a number of advantages, as detailed in the aforementioned references, particularly in the context of internal combustion reciprocating engines.
As also detailed in the aforementioned references, ignition delay is believed to occur due to the need for the fuel molecules to first be dissassociated into radicals in order to combine with oxygen molecules, which themselves must be dissassociated for oxidation to occur. In a typical combustion process, an ignition device such as a spark plug causes a localized increased concentration of fuel radicals in a fuel-air mixture, sufficient for initiation of combustion. The release of heat from that localized combustion in turn causes additional dissassociation of adjacent fuel molecules to enable combustion to progagate through the entire charge of fuel-air mixture.
As discussed in the aforementioned U.S. Pat. No. 4,448,176, if there is a pretreatment of the fuel such as to cause a dissassociation of a critical proportion of molecules in each quantity of fuel, much higher than the proportion occurring at ambient temperatures, there is an "activation" of the fuel yielding substantially instantaneous ignition and combustion.
In order that such an increased, critical proportion of fuel molecules be dissassociated into radicals, energy must be expended to bring this proportion of the fuel molecules to the relatively high energy state corresponding to the dissassociated condition of the fuel molecules.
As described in the aforementioned U.S. Pat. No. 4,448,176, if fuel is heated to relatively elevated temperatures in excess of 1000.degree. F., this causes such critical proportion of fuel molecules to be dissassociated to form radicals, since such proportion of the fuel molecules is thereby brought to a high energy state.
In co-pending application Ser. No. 812,863 filed on Dec. 26, 1985, there is described a method and system for heating of the fuel to such elevated temperatures by a regenerative heat exchange process, in which the fuel is circulated through a vessel disposed directly in the combustion chamber, with the combustion chamber insulated to retain heat therein. This arrangement, when combined with preheating of the fuel, as with an exchanger in the engine exhaust system, is able to heat the fuel to such sufficiently elevated temperatures, that upon injection into the combustion chamber hypergolic combustion will result.
Alternatively, catalysis is utilized to augment the effect of heating to achieve fuel activation.
A disadvantage of regeneratively heating the fuel is the tendency for cracking of the fuel molecules at high temperatures, and the resultant formation of coke, tending to clog the fuel flow passages. It has been discovered that if the fuel is sustained at the elevated temperatures for only very short time periods, this will avoid this problem.
Also, while such aforementioned regenerative heating method and system will efficiently heat the fuel to such elevated temperatures, a relatively complex fuel circulation system is necessitated and the tendency for coke formation is higher due to the longer times required to achieve heating by heat exchange with the products of combustion produced in the prior combustion cycles.
In co-pending application Ser. No. 813,888 filed on Dec. 26, 1986, a method of compressive heating of vaporized fuel is disclosed to achieve activated fuel enabling hypergolic combustion, in which catalysis is alternatively utilized to augment the effect achieved by compression heating alone.
Another method of pretreatment of fuel is disclosed in Ser. No. 446,796, filed on Dec. 3, 1982, now U.S. Pat. No. 4,587,475 in which a corona discharge is utilized to activate fuel passed therethrough prior to combustion, to achieve activation of the fuel molecules by the formation of radicals.
There has also been disclosed in co-pending application Ser. No. 813,892 filed on Dec. 26, 1986, a method of activating fuel by irradiation with ultraviolet radiation to achieve the level of activation of the fuel enabling hypergolic combustion.
In SAE paper 800264 entitled "Gasification of Diesel Fuel for a Low Emission, High Efficiency Engine System"; in Siemens Forsch.-u. Entwickl.-Ber.Bd.6 (1977) Nr. 5, entitled "Autothermal Gasification of Liquid Hydrocarbons by Partial Oxidation"; and Siemens Forsch.-u. Entwickl.-Ber.Bd.7(1978) Nr. 2 entitled "Compact Gas Generator for Fuel Gasification Aboard Motor Vehicles", in each reference there is disclosed a method of gasifying liquid hydrocarbon fuels.
This method involves partial oxidation of the fuel in the presence of a catalyst to form a high temperature gas, rich in hydrogen and hydrocarbon radicals. This gas is described as being useable as a fuel for an internal combustion engine if it is cooled to room temperature and mixed with air as in a conventional spark ignited engine. It was found that partial oxidation using catalysis and the proper air-fuel ratios resulted in elimination or a great reduction in coke formation in the catalytic reactor as well as in the fuel delivery lines.
In the aforementioned processes, however, the heat generated by this partial oxidation is extracted from the hydrogen rich gas prior to its combustion in an internal combustion engine.
This increases the complexity of the engine and/or adversely affects the efficiency of the engine, since partial oxidation lowers the heating value of the fuel.
Accordingly, it is an object of the present invention to provide an apparatus and method of achieving hypergolic combustion by partial combustion of vaporized fuel while eliminating or substantially alleviating the tendency for coke formation.
It is another object of the present invention to provide an apparatus and method for partial combustion pretreatment of fuel which does not require regenerative extraction of the heat generated by the partial oxidation, nor resulting in a loss of engine efficiency by loss of a portion of the heat energy which is capable of being generated by the fuel.